This invention relates to a fuel vapor emission control.
An engine comprises a fuel vapor emission control device wherein the fuel vapor generated in a fuel tank when the engine has stopped is first adsorbed by activated carbon in a canister, the fuel adsorbed by the activated carbon is desorbed using a manifold vacuum under a predetermined running condition after the engine starts, and the fuel vapor is led into an intake passage downstream of a throttle valve to be burnt.
JP-A-H6-264832 published by the Japanese Patent Office in 1994 discloses a technique wherein, in a fuel vapor emission control device, to perform an effective purge rate setting and maintain a large purge flowrate, the purge rate is set according to an integrated flowrate. The purge rate is the ratio of the purge flowrate and intake air flowrate. Specifically, as the integrated value of the purge flow rate increases, the purge rate is increased.
However, in this aforesaid prior art, the purge rate relative to the integrated purge flowrate was set to its optimum value in the state where fuel is adsorbed up to the maximum capacity by the canister (full charge state), to prevent oversupply of fuel vapor due to purge. As a result, when the canister is fully charged, the optimum purge rate can be set, but when the adsorption amount of the canister is low, the optimum purge rate cannot be set. In other words, when the canister adsorption amount is small, in the prior art, a small purge rate was set based on the full charge state regardless of the fact that a large purge rate could actually be set in practice. Further, as a fixed purge rate was always set relative to the integrated purge flowrate, it cannot be dealt with when a purge gas of higher concentration than that expected was supplied.
It is therefore an object of this invention to perform a large amount of purge and set an optimum purge rate in a fuel vapor emission control device without worsening the combustion stability of the engine and increasing exhaust gas emissions.
In order to achieve above object, this invention provides a fuel vapor emission control device of an engine which is air-fuel ratio feedback controlled, comprising a canister which adsorbs fuel vapor generated in a fuel tank of the engine, a purge passage which connects the canister and an intake passage of the engine, a purge valve which opens and closes the purge passage, and a controller functioning to compute a target air-fuel ratio feedback deviation, which is the deviation between a target value of the air-fuel ratio feedback correction coefficient and a basic value of the air-fuel ratio feedback correction coefficient, compute an actual air-fuel ratio feedback deviation which is the deviation between an actual air-fuel ratio feedback correction coefficient and the basic value of the air-fuel ratio feedback correction coefficient, set a target purge rate according to the difference between the target air-fuel ratio feedback deviation and the actual air-fuel ratio feedback deviation, and drive the purge valve so that the purge rate becomes the target purge rate.